1. Field of the Invention
This invention relates to metal imaged plates produced by physical development and to processes of making such plates and using them for printing.
2. Description of the Prior Art
A number of printing plates have been developed in the prior art which have the capability of reproducing a continuous tone image. The first printing plate to be used in this type of printing was the collotype plate. This printing process relied on a hydrophilic colloid which became differentially hardened in the exposed areas of the plate. These areas became ink receptive in proportion to the degree of hardening. One of the main drawbacks of this printing plate was the short run length and the lack of repeatability.
More recently printing plates which have been utilized for reproducing continuous tone images are photopolymers and diazonium salts. These printing plates also suffer from a lack of repeatability particularly for longer run lengths. Another serious disadvantage of these prior art printing plates is that they are generally high contrast systems having the capability of recording and printing only a very short tonal range. Therefore, considerable information is lost from most continuous tone images which are lower contrast and contain an extended tonal range.
Because of these difficulties with most continuous tone printing plates, conventional printing is performed by first screening the print to convert the continuous tone image into a pattern of dots called a halftone image. This halftone image is then used to expose the printing plate. The halftone process which is necessary in most commercial printing, however, does degrade the image and introduces problems in the reproduction process. For example, the halftone image may be unable to resolve the fine details in the picture because the dot structure is larger than the fine detail.
Additional advantages of printing plates which can reproduce continuous tone prints directly (screenless printing) over conventional halftone printing are (1) eliminates moire-effects caused by interference between (a) superimposed halftone patterns and (b) subject and halftone patterns are eliminated, (2) there is a greater ability to produce a higher saturation of pastel colors, and (3) the step of making the halftone is eliminated. Thus halftone printing has been looked upon as a necessary evil by printers because of the additional step required in the printing process, the degradation of the quality of the reproduction, and also the extra time and specialized equipment and materials required.